D,L-aspartic acid is useful for the production of polysuccinimide and polyaspartic acid. Poly-alpha,beta-D,L-aspartic acid, copolymers of poly-alpha,beta-D,L-aspartic acid, and their salts are generally water soluble, biodegradable polymers useful in a variety of applications. Such applications include detergent formulations, water treatment, oral hygiene products, pharmaceuticals, agricultural applications, oil production, cosmetics, and personal care products, such as hair and skin care products.
Polyaspartates can be manufactured by polymerization of D,L-aspartic acid and L-aspartic acid, or by polymerization of maleic acid in the presence of ammonia. The polyaspartates derived from D,L-aspartic acid and L-aspartic acid are generally useful in a wide variety of applications. Such applications include uses as mineral scale inhibitors in water treatment; scale and corrosion inhibitors in oil production; humectants in personal care products; and, nutrient absorption enhancers in agriculture. However, the polyaspartates derived from maleic acid and ammonia are not suitable for use in many applications due to limitations on molecular weight or color.
L-aspartic acid is the only form of aspartic acid which is currently available on a large commercial scale. L-aspartic acid is used in the manufacture of the artificial sweetener, aspartame. The manufacture of L-aspartic acid involves the use of fermentation or other biotechnology processes and can be expensive.
The production of D,L-aspartic acid by various methods is known in the art. Polish Pat. No. 133691 and Polish Pat. No. 143183 disclose the production of D,L-aspartic acid by reacting maleic acid and ammonia in the presence of a large amount of ammonium chloride at an elevated temperature and pressure. The resulting diammonium salt of aspartic acid is acidified with hydrochloric acid to afford a 60% yield of D,L-aspartic acid. However, for every mole of aspartic acid produced by the disclosed process, four moles of ammonium chloride are produced as waste. Inclusion of a large amount of ammonium chloride and the acidification with hydrochloric acid are undesirable process expedients because of the added waste produced, and because of the corrosive nature of such additives which requires special process equipment.
East German Pat. No. DD126075 discloses a method for the manufacture of D,L-aspartic acid in which maleic anhydride is neutralized with aqueous ammonia at a normal pressure and an elevated temperature. An additional solvent, N-formylacetamide, is added and the solution is heated at 120-130 degrees C. Hydrochloric acid is then added to the reaction mixture to yield D,L-aspartic acid. However, the N-formylacetamide is destroyed in this process, creating undue waste and added expense. Inclusion of an additional solvent, such as N-formylacetamide, in the method of the present invention would have a deleterious effect because of the added waste and expense. In addition, carrying out the method of the present invention at a normal, rather than an elevated pressure, would have an adverse effect on the formation of the reaction product.
U.S. Pat. No. 4,839,461 to Boehmke teaches the production of polyaspartic acid by reacting maleic acid and ammonia. The products can be converted into D,L-aspartic acid by treatment with hydrochloric acid. The process of Boehmke teaches the addition of a metal hydroxide or ammonium hydroxide, the reaction of maleic acid and ammonia in a molar ratio of 1:1-1.5, and the production of monoammonium salt.
In addition, methods of manufacturing polymers of aspartic acid are known. U.S. Pat. No. 5,219,952 to Koskan et al. teaches the production of high molecular weight polysuccinimide and high molecular weight polyaspartic acid from maleic anhydride and ammonia. U.S. Pat. No. 5,292,858 to Wood teaches the production of copolymers of polyamino acids. U.S. Pat. No. 5,373,088 to Koskan et al. teaches the production of polyaspartic acid from maleic acid and ammonia. U.S. Pat. No. 5,491,213 to Batzel teaches the production of polysuccinimide by reacting an unsaturated dicarboxylic acid or anhydride with a particulate ammonium salt.
The known methods for producing D,L-aspartic acid have several disadvantages. They are more costly because additional reactants and more expensive plant equipment must be used, thus increasing the cost. They are more wasteful because various end products and by-products are discarded rather than recycled back into the process.
Therefore a need exists for a more efficient and low-cost method of producing D,L-aspartic acid that simultaneously yields a high amount of product. The method of the present invention overcomes the problems associated with the known methods of making D,L-aspartic acid by recovering and recycling excess ammonia used in the method, as well as recovering and recycling various additional by-products for re-use in the method. Such recycling of products reduces waste and provides for a more efficient method of production. Thus, the present invention provides an efficient, cost-effective method for the manufacture of D,L-aspartic acid.